Warming is increasing oceans' dead zones, study finds

Oxygen levels fall, threatening to destroy sea life

Published 5:30 am, Friday, May 2, 2008

WASHINGTON — Low oxygen zones where sea life is threatened or cannot survive are growing as the oceans are heated by global warming, researchers warn.

Oxygen-depleted zones in the central and eastern equatorial Atlantic and equatorial Pacific oceans appear to have expanded over the last 50 years, researchers report in today's edition of the journal Science.

Low oxygen zones in the Gulf of Mexico and other areas also have been studied in recent years, raising concerns about the threat to sea life.

Continued expansion of these zones could have dramatic consequences for both sea life and coastal economies, said the team led by Lothar Stramma of the University of Kiel in Germany.

"Many species will lose their deep habitat, meaning competition will become stronger in the remaining favorable habitat, and increased vulnerability to predation will likely occur," said Whitney, who was not part of Stramma's team.

He said the most rapid oxygen declines he has seen have occurred in the subarctic Pacific Ocean, and fish and crab kills have been reported in the last few years off Oregon.

The general pattern is for colder ocean waters in the north and south to absorb oxygen, cool and sink below the surface to then flow toward the equator, Johnson explained.

Along the way, organic matter drifts down into the deeper water and its decay uses up some of this oxygen. The oxygen balance depends on this movement and the amount of oxygen reaching the warmer waters, Johnson said, and this can be reduced if less is absorbed and moved in the deep currents.

"That means that eventually, at the end of the line, there will be less oxygen," he said.

In cold surface water, oxygen levels can reach as high as 300 to 400 micromols per kilogram, Johnson said. A mol of a gas such as oxygen occupies a volume of just under six gallons, and a micromol is one-thousandth of that. A kilogram of water is the amount that would weigh 2.2 pounds.

Dissolved oxygen varies widely in the oceans, and sea life becomes stressed when it reaches between 60 and 120 micromols per kilogram.

The researchers found concentrations as low as 10 in parts of the eastern Pacific and the northern Indian Ocean and larger areas in the Atlantic and Pacific were below 150.